1. Field of the Invention
The present invention relates to surface acoustic wave devices, such as surface acoustic wave resonators or surface acoustic wave filters, and manufacturing methods therefor, and more particularly, relates to an electrode structure of a surface acoustic wave device and a forming method therefor.
2. Description of the Related Art
As has been well known, surface acoustic wave devices are electronic elements including a surface acoustic wave in which mechanical vibration energy is concentrated only in the vicinity of surfaces of a solid material and is then propagated. In addition, the surface acoustic wave devices are each generally composed of a piezoelectric substrate having piezoelectric properties and electrodes, such as interdigital electrodes and/or grating electrodes, disposed on the piezoelectric substrate for processing electrical signals and surface acoustic waves.
In the surface acoustic wave devices described above, as an electrode material, aluminum (Al) having a low electrical resistivity and a low specific gravity or an alloy thereof has been used.
However, since Al has poor stress migration resistance, when a large electrical power is applied thereto, hillocks and/or voids are formed in the electrodes, and short-circuiting or disconnection of the electrodes may occur in some cases, resulting in breakage of the surface acoustic wave device.
In order to solve the problems described above, a method for improving electrical power resistance has been disclosed in Japanese Unexamined Patent Application Publication No. 7-162255 (patent publication 1) in which the crystal orientation is improved by an ion beam sputtering method used as a method for forming electrodes.
In addition, another method for improving electrical power resistance has been proposed in Japanese Unexamined Patent Application Publication No. 3-48511 (patent publication 2) in which an Al crystal is oriented in a predetermined direction by an epitaxial growth method.
Japanese Unexamined Patent Application Publication No. 6-6173 (patent publication 3) has disclosed that electrical power resistance of electrodes can be improved as crystal grain size is decreased.
Furthermore, in “Technical Handbook of Surface Acoustic Wave Device” edited by the 150th Committee on Technology of Surface Acoustic Wave Device of Japan Society for the Promotion of Science, published by Ohmsha, Ltd., p. 267 (non-patent publication 1), a phenomenon has been disclosed in which the electrical power resistance is improved when copper (Cu) is added to Al.    Patent publication 1: Japanese Unexamined Patent Application Publication No. 7-162255    Patent publication 2: Japanese Unexamined Patent Application Publication No. 3-48511    Patent publication 3: Japanese Unexamined Patent Application Publication No. 6-6173    Non-patent publication 1: “Technical Handbook of Surface Acoustic Wave Device” edited by the 150th Committee on Technology of Surface Acoustic Wave Device of Japan Society for the Promotion of Science, published by Ohmsha, Ltd., p. 267.
However, by the traditional techniques disclosed in patent publications 1 and 3, recent higher frequency and larger electrical power requirements cannot satisfactorily be fulfilled, and hence, when the techniques described above are used in high-frequency or large electrical power applications, insufficient electrical power resistance becomes a serious problem.
In addition, according to the traditional technique disclosed in patent publication 2, an epitaxial film having superior crystallinity can be actually grown only on a quartz substrate. However, on a substrate composed of a piezoelectric crystal, such as LiTaO3 or LiNbO3, used for filters which have superior piezoelectric properties and are advantageously used in a broad band, it has been difficult to grow an epitaxial film having superior crystallinity by the technique disclosed in patent publication 2, and as a result, the traditional technique described above cannot practically be applied to a surface acoustic wave device including a LiTaO3 or LiNbO3 substrate.
According to the traditional technique disclosed in non-patent publication 1, by adding Cu to Al, the electrical power resistance can actually be improved. However, a level of this improvement has not been satisfactory in practice.